1. Field of the Invention
This invention relates to a continuous process for producing a vinyl acetate-ethylene copolymer emulsion.
2. Description of the Prior Art
The earliest work in preparing vinyl acetate-ethylene emulsions, which are suited as adhesive compositions for rug backing applications, woven and nonwoven goods, and impregnating applications, seems to have been done by Perrin in U.S. Pat. No. 2,200,429. The earliest work generally resulted in low ethylene concentrations, e.g., 2-5% by weight.
Roedel in U.S. Pat. No. 2,703,794, described a continuous process for producing vinyl acetate-ethylene emulsions by carrying out the polymerization in the presence of tertiary butyl alcohol, an emulsifying agent and a redox catalyst system comprising a peroxygen compound and a reducing agent. Essentially, the process comprised a batch reaction with continuous addition and withdrawal of the polymer recipe from the reactor. Some of the difficulties in this processes were substantial wall fouling and latex irregularities in terms of copolymer particle size.
In British Pat. No. 991,550, a batch polymerization process is described in which vinyl acetate-ethylene copolymers could be produced having an ethylene concentration of 10-15% by weight. In that process, vinyl acetate was continuously introduced to the reactor while maintaining an ethylene pressure of about 20 atmospheres. A surface active agent and protective colloid were included to enhance product stability and polymerization was effected using a redox catalyst system.
U.S. Pat. No. 3,708,388 discloses a batch process for producing vinyl acetate-ethylene copolymer emulsions having superior adhesion to other vinyl acetate-ethylene copolymer emulsions and having higher ethylene content. It was noted that the earlier continuous processes were difficult to operate and achieve uniform ethylene concentration and consistent particle size. In the U.S. Pat. No. 3,708,388, it was disclosed that higher ethylene concentrations could be achieved by polymerizing the vinyl acetate and ethylene in the presence of a protective colloid, however, the adhesive quality of the emulsion was increased by permitting an equilibrium to be achieved between the vinyl acetate and ethylene prior to initiating polymerization. It was also reported that when using polyvinyl alcohol protective colloid greater adhesive quality was achieved.
U.S. Pat. No. 4,035,329 discloses a process for the continuous production of an ethylene-vinyl acetate copolymer dispersion. As noted in that patent, the Roedel U.S. Pat. No. 2,703,794 process does not proceed at a satisfactory speed or in a uniform manner. To overcome some of the disadvantages in prior art continuous processes, the patent notes that the polymerization should be effected in the presence of an emulsifier and/or protective colloid, a free radical water soluble redox catalyst system comprising a reducing agent and oxidizing agent with the molar ratio of reducing agent to oxidizing agent being at least 3:1. In addition, the monomers should be added to the polymerization zone at a rate such that the concentration of unreacted monomers does not exceed 15% by weight of the total weight of the reaction mixture. By employing this particular technique, it is reported that wall fouling is reduced and polymerization is carried out at a satisfactory commercial rate.